1. Field of the Invention
The present invention relates to focusing bias adjusting apparatus and method in an optical recording medium playing apparatus.
2. Description of the Related Art
To accurately read recording information from an optical disc such as CD (Compact Disc), DVD (Digital Versatile Disc), or the like, it is necessary to vertically irradiate a reading beam onto a recording surface of the optical disc. If a warp occurs in the optical disc itself or an error of a mechanism system is large, however, the reading beam cannot be vertically irradiated onto the recording surface of the optical disc and information reading precision deteriorates.
A recording medium playing apparatus for reproducing the recording information from the optical disc is, therefore, provided with a tilt servo control apparatus for detecting a tilt occurring between a pickup as information reading means and the optical disc, inclining the whole pickup by an angle corresponding to the detected tilt, or performing a tilt correcting process corresponding to the tilt to a read signal read out by the pickup, thereby suppressing the deterioration of the information reading precision.
As a tilt servo control apparatus, an apparatus using a liquid crystal panel inserted onto an optical axis in order to execute the tilt correction is known (for example, JP-A-11-3531). In the case of the apparatus, the liquid crystal panel is divided into a plurality of regions and a phase difference is given to a laser beam passing through each region of the liquid crystal panel so as to maximize a level of the read signal derived by the pickup.
Since the irradiation laser beam needs to be focused onto the recording surface of the optical disc in order to accurately read the recording information from the optical disc, an optical disc player is provided with a focusing servo control apparatus for generating a focusing error signal on the basis of each photodetecting level of a plurality of split photodetectors of the pickup and controlling a position in the optical axial direction of an objective lens so as to reduce a level of the focusing error signal. In the focusing servo control apparatus, generally, the focusing error signal is not set to a reference level (for example, 0) in an in-focus state because of an error of an optical system but includes a DC component as an offset component in the focusing error signal. A focusing bias adjustment, therefore, for forcibly setting the offset component in the focusing error signal to 0 is executed so that the focusing error signal is set to the reference level in the in-focus state.
The focusing bias adjustment is generally executed as an initial operation just after the optical disc was set to the disc player. Since the tilt servo control operation is performed by the tilt servo control apparatus even during the playback of the disc, however, if the tilt correction is executed by the tilt servo control operation, since the focusing bias adjustment executed as an initial operation is based on the photodetecting level of each photodetector of the pickup before the tilt correction is executed, there is such a problem that an adjustment value is deviated from a true adjustment value as a result and is not proper. The problem also similarly occurs in the case where an apparatus in which an optical device other than the liquid crystal panel is inserted into the optical system of the pickup so as to maximize the level of the read signal derived by the pickup is used as a tilt servo control apparatus.